The discharge of hydrocarbons from petrol automobile exhaust is minimum when the vehicle is cruising.

Explanation:

When a petrol automobile is running, it emits various pollutants, including hydrocarbons, carbon monoxide, nitrogen oxides, and particulate matter. Hydrocarbons are compounds consisting of carbon and hydrogen atoms and are one of the primary components of gasoline.

Factors affecting hydrocarbon emissions:

There are several factors that influence the discharge of hydrocarbons from automobile exhaust, including engine speed, load, and temperature. The combustion process in the engine generates hydrocarbons, and the efficiency of the emission control systems can vary depending on the operating conditions of the vehicle.

Engine Speed:
- At idle, the engine speed is low, resulting in incomplete combustion of fuel, which leads to higher hydrocarbon emissions.
- During acceleration, the engine speed increases rapidly, causing a rich fuel mixture and incomplete combustion, resulting in elevated hydrocarbon emissions.
- During deceleration, the engine speed decreases, and the fuel supply is cut off or reduced. This can cause unburned fuel to be released into the exhaust system, leading to increased hydrocarbon emissions.

Cruising:
- When a vehicle is cruising, the engine operates at a relatively constant speed and load.
- At this point, the fuel-air mixture is optimized, resulting in complete combustion of fuel, thereby minimizing hydrocarbon emissions.
- The emission control systems, such as the catalytic converter, are also designed to operate efficiently at cruising speeds, further reducing hydrocarbon emissions.

Conclusion:
In conclusion, the discharge of hydrocarbons from petrol automobile exhaust is minimum when the vehicle is cruising. At cruising speeds, the engine operates at a consistent speed and load, allowing for optimized fuel combustion and efficient operation of emission control systems. This results in reduced hydrocarbon emissions compared to idling, accelerating, or decelerating conditions.